Generally, a conventional optical memory has employed a process of entering light onto a reflective surface formed on a substrate and measuring the intensity of a resulting reflected light to read the information recorded in the reflective surface. For example, as in CDs, there has been used a process of forming a number of microstructures (cells) on a circular rotatable substrate and reducing the size of the microstructure to increase a recording density. Recently, instead of the process as in the CD, there has been proposed a process of entering light onto a multilayer film deposited on a substrate and measuring polarization of a resulting reflected light to read the information recorded in the multilayer film ([1] R. Jansson, H. Arwin, I. Lundstrom, Applied Optics, 33, 6843 (1994), [2] R. Jansson, R. Wigren, K. Jarrendahl, I. Lundstrom, H. Arwin, Optics Communications, 104, 277 (1994)).
The inventors have also proposed a new memory medium using a transparent substrate, characterized by measuring polarization of a transmitted light, instead of a reflected light (Masato Tazawa, Jin Ping, Japanese Patent Application No. 2001-230470). In this process, a typical parameter representing polarization of light is an angle referred to as “ellipsometric parameter” and generally expressed by symbols Δ and Ψ. The polarizations of reflected light and transmitted light are defined by using the following formulas (1) and (2), respectively:ρ=rp/rs=tan ΨeiΔ  (1),wherein ρ is a complex reflection coefficient ratio, and rp and rs are complex reflection coefficients of p-polarization and s-polarization, respectively; andρ=tp/ts=tan ΨeiΔ  (2),wherein ρ is a complex transmission coefficient ratio, and tp and ts are complex transmission coefficients of p-polarization and s-polarization, respectively.
The parameters defining the complex reflection coefficient ratio and the complex transmission coefficient ratio, or the values Δ and Ψ, are determined by the structure of the multilayer film. Thus, the structure of the multilayer film can be learnt by measuring the values Δ and Ψ. That is, it means that if necessary information is formed on the substrate as the structure of the multilayer film in advance, the structure of the multilayer film will function as a memory. Specifically, the information recorded as the structure of the multilayer film can be read by measuring the values Δ and Ψ and subjecting the measured values to appropriate information processing. Based on the above process, a number of microstructures (cells) can be formed on a circular rotatable substrate such as a CD to store information in a high capacity.
FIG. 1 shows one example of a multilayer film structure formed by the conventional process of depositing a multilayer film on a substrate. This structure includes aluminum and molybdenum films having a thickness of 5 nm, and the films are formed on a silicon substrate in a 4-layer structure. In FIG. 1, the aluminum region is shown by a dark gray color, and the molybdenum region is shown by a light gray color. If the aluminum and molybdenum regions are associated, respectively, with 1 and 0 of binary numbers, and the lower layer is defined as a higher bit in advance, the cells will have information such as (0000), (0001), (0010), (0011), (0100), (0101), (0110), (0111) - - - , from the left side of the figure.
Then, the values Δ and Ψ of the cells in the multilayer film can be calculated through a conventional technique for thin film optics, as shown in FIG. 2, wherein the horizontal and vertical axes represent Δ and Ψ, respectively, and each point has the 4-digit binary information specified adjacent thereto. In this example, incident light had an incident angle of 70 degrees and a wavelength of 632.8 nm. Thus, a 4-bit optical memory can be constructed by forming a thin film having the above structure during writing of information, and measuring the values Δ and Ψ and associating them with respective 4-digit binary numbers during reading of the information.
The related technical publications include [1] R. Jansson, H. Arwin, I. Lundstrom, Applied Optics, 33, 6843 (1994), and [2] R. Jansson, R, Wigren, K. Jarrendahl, I. Lundstrom, H. Arwin, Optics Communications, 104, 277 (1994).
The above multilayer film is formed using a thin-film forming method such as a sputtering method or an MBE method. A measurement method of the values Δ and Ψ is known as ellipsometry (see the above publications [1] R. Jansson, H. Arwin, I. Lundstrom, Applied Optics, 33, 6843 (1994), and [2] R. Jansson, R. Wigren, K. Jarrendahl, I. Lundstrom, H. Arwin, Optics Communications, 104, 277 (1994)). The parameter for expressing the polarization is not limited to the values Δ and Ψ, but any other suitable value, such as well known cos Δ and tan Ψ, or two or more kinds of voltage or current values which can be read directly from a detector, may be used.
However, the multilayer film structure employed as an information-recording medium in the conventional optical memory precludes a possibility of producing the optical memory through a stamping process used in producing conventional CDs, which undesirably leads to increased production cost. In addition, incident light has to be entered at an incident angle of about 70 degrees to detect the values Δ and Ψ, and consequently the surface of the memory is irradiated with light having a length three times greater than the beam width of the incident light. Thus, the size of the memory cell cannot be reduced beyond the above length, which undesirably restricts a recording capacity per unit area.
As described above, while the conventional optical memory formed, for example, with a 4-layer film and adapted to read the values Δ and Ψ therefrom can record information in a single cell in 16 of different states, and has a possibility of achieving a high-capacity optical memory, it involves problems of an increased production cost due to inapplicability of the conventional production process, and of restriction in reducing the size of the cell itself.